Magnetoresistive random access memory (MRAM) devices are solid state, non-volatile memory devices which make use of the giant magnetoresistive effect. A conventional MRAM device includes a column of first electrical wires, referred to as word lines, and a row of second electrical wires, referred to as bit lines. An array of magnetic memory cells, located at the junctions of the word lines and bit lines, is used to record data signals.
A typical magnetic memory cell comprises a hard magnetic layer, a soft magnetic layer, and a non-magnetic layer sandwiched between the hard magnetic layer and the soft magnetic layer. The hard magnetic layer has a magnetization vector fixed in one direction. The orientation of the magnetization vector does not change under a magnetic field applied thereon. The soft magnetic layer has an alterable magnetization vector under a magnetic field applied thereon, that either points to the same direction, hereinafter “parallel alignment”, or to the opposite direction, hereinafter “antiparallel alignment”, of the magnetization vector of the hard magnetic layer. Since the resistances of the magnetic memory cell in the “parallel alignment” status and the “antiparallel alignment” status are different, the two types of alignment status can be used to record the two logical states—the “0”s or “1”s of a data bit.
In a writing operation, an electric current passes through the word line and the bit line adjacent to the memory cell. When the electric current reaches a certain threshold, a magnetic field generated by the electric current will switch the orientation of the magnetization vector of the soft magnetic layer. As a result, the magnetization vector of the hard magnetic layer and the soft magnetic layer will be changed from one type of alignment, e.g. “parallel alignment”, to the other type of alignment, e.g. “antiparallel alignment”, so that a data signal in form of one data bit can be recorded in the memory cell.
In MRAM structure design, a lower writing power dissipation and a higher cell density are most desired. Unfortunately, a reduction of cell size, i.e. an increase in cell density, will lead to a reduction in the available energy (KuV) to store the bit message. Further, the error rate increases very rapidly as the cell size scales down. However, in order to reduce the error rate, high anisotropy energy is required to overcome thermal fluctuations. Hard magnetic material has higher anisotropy energy compared with soft magnetic material, but in that case a higher writing current is required. The higher anisotropy energy results in higher writing current density, which unfortunately has the disadvantage of electro-migration.
In order to reduce the writing current for a high coercitivity MRAM, thermally assisted MRAMs are disclosed in U.S. Pat. No. 6,385,082, U.S. patent application 20020089874, JP patent application 2002208680, and JP patent application 2002208681. Un-pinned ferromagnetic materials, in which the coercitivity decreases sharply as temperature increases, are used for the recording layer in the MRAMs disclosed therein.
In order to increase the thermal stability of the magnetic memory cell, recently, a Curie point written MRAM has been proposed to improve the stability of MRAM, as described in U.S. Pat. No. 6,535,416, and in a paper by R. S. Beech et al.: “Curie point written magnetoresistive memory” in J. Appl. Phys. 87, No. 9, pp. 6403-6405, 2000. In the Curie point written MRAM structure, a single pinned layer is used as storage layer. The pinned layer has a higher anisotropy than an unpinned layer. The use of the pinned layer for information storage provides improved thermal stability, allowing the cell size to be reduced before thermal instability becomes a limiting factor.
In order to increase the MRAM cell density, the MRAM structure can be simplified as mentioned in U.S. Pat. No. 6,597,618, U.S. Pat. No. 6,341,084, U.S. Pat. No. 6,317,375 and U.S. Pat. No. 6,259,644.
In analogy to a conventional single-layered magnetic media, thermal stability can be improved by introduction of antiferromagnetically coupled magnetic layers as the magnetic memory cell size decreases further (see, e.g., E. E. Fullerton et al.: “Antiferromagnetically coupled magnetic media layers for thermally stable high-density recording” in Appl. Phys. Lett. 77, No. 23, p. 3806, 2000).